onyx vs sardonyx

onyx and sardonyx both are nouns.

Word	Noun	Adjective	Verb	Adverb
onyx	Yes	No	No	No
sardonyx	Yes	No	No	No

As nouns, sardonyx is a hyponym of onyx; that is, sardonyx is a word with a more specific, narrower meaning than onyx:

onyx: a chalcedony with alternating black and white bands; used in making cameos
sardonyx: an onyx characterized by parallel layers of sard and a different colored mineral

onyx (noun)	sardonyx (noun)
a chalcedony with alternating black and white bands; used in making cameos	an onyx characterized by parallel layers of sard and a different colored mineral